Electro-optical device and electronic device

ABSTRACT

An object of the present invention is to provide an EL display device having a high operation performance and reliability. 
     The switching TFT  201  formed within a pixel has a multi-gate structure, which is a structure which imposes an importance on reduction of OFF current value. Further, the current control TFT  202  has a channel width wider than that of the switching TFT to make a structure appropriate for flowing electric current. Moreover, the LDD region  33  of the current control TFT  202  is formed so as to overlap a portion of the gate electrode  35  to make a structure which imposes importance on prevention of hot carrier injection and reduction of OFF current value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electro-optical device, typically anEL (electroluminescence) display device formed by a semiconductorelement (an element using a semiconductor thin film) made on asubstrate, and to electronic equipment (an electronic device) having theelectro-optical device as a display (also referred to as a displayportion).

2. Description of the Related Art

Techniques of forming a TFT on a substrate have been widely progressingin recent years, and developments of applications to an active matrixtype display device are advancing. In particular, a TFT using apolysilicon film has a higher electric field effect mobility (alsoreferred to as mobility) than a TFT using a conventional amorous siliconfilm, and high speed operation is therefore possible. As a result, itbecomes possible to perform pixel control, conventionally performed by adriver circuit external to the substrate, by a driver circuit formed onthe same substrate as the pixel.

This type of active matrix display device has been in the spotlightbecause of the many advantage which can be obtained by incorporatingvarious circuits and elements on the same substrate in this type ofactive matrix display device, such as reduced manufacturing cost, smallsize, increased yield, and higher throughput.

Switching elements are formed by a TFT for each of the pixels in theactive matrix display device, current control is performed by driverelements using the switching elements, and an EL layer(electroluminescence layer) is made to emit light. A typical pixelstructure at this time is disclosed in, for example, in FIG. 1 of U.S.Pat. No. 5,684,365 (Japanese Patent Application Laid-open No. Hei8-234683).

As shown in FIG. 1 of the US patent, a drain of a switching element (T1)is connected to a gate electrode of a current control element (T2), andis also connected in parallel to a capacitor (Cs). The gate voltage ofthe current control element (T2) is maintained by the electric chargestored in the capacitor (Cs).

Conversely, when the switching element (T1) is in the non-selectedstate, the electric charge leaks through the switching element (T1) ifthe capacitor (Cs) is not connected (the flow of current at this pointis referred to as off current), and the voltage applied to the gateelectrode of the current control element (T2) cannot be maintained. Thisis a problem which cannot be avoided when the switching element (T1) isformed by a transistor without forming the capacitor. However, thecapacitor (Cs) is formed within the pixel, and therefore this becomes afactor in reducing the effective luminescence surface area (effectiveimage display area) of the pixel.

Further, it is necessary for a large current to flow in the currentcontrol element (T2) in order to allow the EL layer to emit light. Inother words, the performance required for the TFT is entirely differentbetween the switching element and the current control element. In such acase, it is difficult to ensure the performance required by all of thecircuits and element with only one kind of TFT structure.

SUMMARY OF THE INVENTION

In view of the above conventional technique, an object of the presentinvention is to provide an electro-optical device having good operationperformance and high reliability, and in particular, to provide an ELdisplay device. Another object of the present invention is to increasethe quality of electronic equipment (an electronic device) having theelectro-optical device as a display by increasing the image quality ofthe electro-optical device.

In order to achieve the above objects, the present invention assignsTFTs having an optimal structure in view of the performance required byelements contained in each pixel of the EL display device. In otherwords, TFTs having different structures exist within the same pixel.

Specifically, an element which places the most importance onsufficiently lowering the value of the off current (such as a switchingelement) is given a TFT structure in which the importance is more onreducing the off current value rather than on high speed operation. Anelement which places the greatest importance on current flow (such as acurrent control element) is given a TFT structure in which theimportance is more on current flow, and on controlling deterioration dueto hot carrier injection, which becomes a conspicuous problem at thesame time, rather than on reducing the value of the off current.

It becomes possible to raise the operating performance of the EL displaydevice, and to increase its reliability, with the present invention byperforming proper use of TFTs on the same substrate, as above. Note thatthe concepts of the present invention are not limited to a pixelportion, and that the present invention is characterized by the point ofbeing able to optimize the TFT structure contained in the pixel portionand in a driver circuit portion for driving the pixel portion.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram showing the cross sectional structure of the pixelportion of an EL display device;

FIGS. 2A and 2B are diagrams showing the top view and the composition,respectively, of the pixel portion of an EL display device;

FIGS. 3A to 3E are diagrams showing manufacturing processes of an activematrix type EL display device;

FIGS. 4A to 4D are diagrams showing manufacturing processes of an activematrix type EL display device;

FIGS. 5A to 5C are diagrams showing manufacturing processes of an activematrix type EL display device;

FIG. 6 is a diagram showing an external view of an EL module;

FIG. 7 is a diagram showing the circuit block structure of an EL displaydevice;

FIG. 8 is an enlarged diagram of the pixel portion of an EL displaydevice;

FIG. 9 is a diagram showing the element structure of a sampling circuitof an EL display device;

FIG. 10 is a diagram showing the composition of the pixel portion of anEL display device;

FIG. 11 is a diagram showing the cross sectional structure of an ELdisplay device;

FIGS. 12A and 12B are diagrams showing the top view and the composition,respectively, of the pixel portion of an EL display device;

FIG. 13 is a diagram showing the cross sectional structure of the pixelportion of an EL display device;

FIG. 14 is a diagram showing the cross sectional structure of the pixelportion of an EL display device;

FIGS. 15A and 15B are diagrams showing the top view and the composition,respectively, of the pixel portion of an EL display device;

FIGS. 16A to 16F are diagrams showing specific examples of electronicequipment;

FIGS. 17A and 17B are diagrams showing external views of an EL module;

FIGS. 18A to 18C are diagrams showing manufacturing processes of acontact structure;

FIG. 19 is a diagram showing the laminate structure of an EL layer;

FIGS. 20A and 20B are diagrams showing specific examples of electronicequipment;

FIGS. 21A and 21B are diagrams showing the circuit composition of thepixel portion of an EL display device;

FIGS. 22A and 22B are diagrams showing the circuit composition of thepixel portion of an EL display device; and

FIG. 23 is a diagram showing the cross sectional structure of the pixelportion of an EL display device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment Mode

FIGS. 1 to 2B are used in explaining the preferred embodiments of thepresent invention. Shown in FIG. 1 is a cross sectional diagram of apixel of an EL display device of the present invention, in FIG. 2A isits top view, and in FIG. 2B is a circuit composition. In practice, apixel portion (image display portion) is formed with a multiple numberof this type of pixel arranged in a matrix state.

Note that the cross sectional diagram of FIG. 1 shows a cross sectioncut along the line A-A′ in the top view shown in FIG. 2A. Common symbolsare used in FIG. 1 and in FIGS. 2A and 2B, and therefore the threefigures may be referenced as appropriate. Furthermore, two pixels areshown in the top view of FIG. 2A, and both have the same structure.

Reference numeral 11 denotes a substrate, and reference numeral 12denotes a base film in FIG. 1. A glass substrate, a glass ceramicsubstrate, a quartz substrate, a silicon substrate, a ceramic substrate,a metallic substrate, or a plastic substrate (including a plastic film)can be used as the substrate 11.

Further, the base film 12 is especially effective for cases in which asubstrate containing mobile ions, or a substrate having conductivity, isused, but need not be formed for a quartz substrate. An insulating filmcontaining silicon may be formed as the base film 12. Note that the term“insulating film containing silicon.” indicates, specifically, aninsulating film that contains silicon, oxygen, and nitrogen inpredetermined ratios such as a silicon oxide film, a silicon nitridefilm, or a silicon oxynitride film (denoted by SiO.sub.xN.sub.y).

Two TFTs are formed within the pixel here. Reference numeral 201 denotesa TFT functioning as a switching element (hereafter referred to as aswitching TFT), and reference numeral 202 denotes a TFT functioning as acurrent control element for controlling the amount of current flowing toan EL element (hereafter referred to as a current control TFT), and bothare formed by an n-channel TFT.

The field effect mobility of the n-channel TFT is larger than the fieldeffect mobility of a p-channel TFT, and therefore the operation speed isfast and electric current can flow easily. Further, even with the sameamount of current flow, the n-channel TFT can be made smaller. Theeffective surface area of the display portion therefore becomes largerwhen using the n-channel TFT as a current control TFT, and this ispreferable.

The p-channel TFT has the advantages that hot carrier injectionessentially does not become a problem, and that the off current value islow, and there are already reports of examples of using the p-channelTFT as a switching TFT and as a current control TFT. However, by using astructure in which the position of an LDD region differs, the problemsof hot carrier injection and the off current value in the n-channel TFTare solved by the present invention. The present invention ischaracterized by the use of n-channel TFTs for all of the TFTs withinall of the pixels.

Note that it is not necessary to limit the switching TFT and the currentcontrol TFT to n-channel TFTs in the present invention, and that it ispossible to use p-channel TFTs for either the switching TFT, the currentcontrol TFT, or both.

The switching TFT 201 is formed having: an active layer comprising asource region 13, a drain region 14, LDD regions 15 a to 15 d, a highconcentration impurity region 16, and channel forming regions 17 a and17 b; a gate insulating film 18; gate electrodes 19 a and 19 b, a firstinterlayer insulating film 20, a source wiring 21, and a drain wiring22.

As shown in FIG. 2A, the present invention is characterized in that thegate electrodes 19 a and 19 b become a double gate structureelectrically connected by a gate wiring 211 which is formed by adifferent material (a material having a lower resistance than the gateelectrodes 19 a and 19 b). Of course, not only a double gate structure,but a so-called multi-gate structure (a structure containing an activelayer having two or more channel forming regions connected in series),such as a triple gate structure, may also be used. The multi-gatestructure is extremely effective in lowering the value of the offcurrent, and by making the switching TFT 201 of the pixel into amulti-gate structure with the present invention, a low off current valuecan be realized for the switching TFT.

The active layer is formed by a semiconductor film containing a crystalstructure. In other words, a single crystal semiconductor film may beused, and a polycrystalline semiconductor film or a microcrystallinesemiconductor film may also be used. Further, the gate insulating film18 may be formed by an insulating film containing silicon. Additionally,a conducting film can be used for all of the gate electrodes, the sourcewiring, and the drain wiring.

In addition, the LDD regions 15 a to 15 d in the switching TFT 201 areformed so as not to overlay with the gate electrodes 19 a and 19 b byinterposing the gate insulating film 18. This structure is extremelyeffective in reducing the off current value.

Note that the formation of an offset region (a region that comprises asemiconductor layer having the same composition as the channel formingregions, and to which a gate voltage is not applied) between the channelforming regions and the LDD regions is more preferable for reducing theoff current value. Further, when a multi-gate structure having two ormore gate electrodes is used, the high concentration impurity regionformed between the channel forming regions is effective in lowering thevalue of the off current.

By thus using the multi-gate structure TFT as the switching TFT 201, asabove, a switching element having a sufficiently low off current valueis realized by the present invention. The gate voltage of the currentcontrol element can therefore be maintained for a sufficient amount oftime (for a period from one selection until the next selection) withoutforming a capacitor (Cs), such as the one stated in the conventionalexample.

Namely, it becomes possible to eliminate the capacitor which causes areduction in the effective luminescence surface area, and it becomespossible to increase the effective luminescence surface area. This meansthat the image quality of the EL display device can be made brighter.

Next, the current control TFT 202 is formed having: an active layercomprising a source region 31, a drain region 32, an LDD region 33, anda channel forming region 34; a gate insulating film 18; a gate electrode35; the first interlayer insulating film 20; a source wiring 36; and adrain wiring 37. Note that the gate electrode 35 has a single gatestructure, but a multi-gate structure may also be used.

As shown in FIGS. 2A and 2B, the drain of the switching TFT 201 iselectrically connected to the gate of the current control TFT 202.Specifically, the gate electrode 35 of the current control TFT 202 iselectrically connected to the drain region 14 of the switching TFT 201through the drain wiring (also referred to as a connection wiring) 22.Further, the source wiring 36 is connected to an electric current supplywiring 212.

A characteristic of the current control TFT 202 is that its channelwidth is larger than the channel width of the switching TFT 201. Namely,as shown in FIG. 8, when the channel length of the switching TFT istaken as L1 and its channel width as W1, and the channel length of thecurrent control TFT is taken as L2 and its channel width as W2, arelational expression is reached in which W2/L2≧5×W1/L1 (preferablyW2/L2≦10×W1/L1). Consequently, it is possible for more current to easilyflow in the current control TFT than in the switching TFT.

Note that the channel length L1 of the multi-gate structure switchingTFT is the sum of each of the channel lengths of the two or more channelforming regions formed. A double gate structure is formed in the case ofFIG. 8, and therefore the sum of the channel lengths L1 a and L1 b,respectively, of the two channel-forming regions becomes the channellength L1 of the switching TFT.

The channel lengths L1 and L2, and the channel widths W1 and W2 are notspecifically limited to a range of values with the present invention,but it is preferable that W1 be from 0.1 to 5 μm (typically between 1and 3 μm), and that W2 be from 0.5 to 30 μm (typically between 2 and 10μm). It is preferable that L1 be from 0.2 to 18 μm (typically between 2and 15 μm), and that L2 be from 0.1 to 50 μm (typically between 1 and 20μm) at this time.

Note that it is preferable to set the channel length L in the currentcontrol TFT on the long side in order to prevent excessive current flow.Preferably, W2/L2≧3 (more preferably W2/L2≧5). It is also preferablethat the current flow per pixel is from 0.5 to 2 μA (better between 1and 1.5 μA).

By setting the numerical values within this range, all standards, froman EL display device having a VGA class number of pixels (640×480) to anEL display device having a high vision class number of pixels(1920×1080) can be included.

Furthermore, the length (width) of the LDD region formed in theswitching TFT 201 is set from 0.5 to 3.5 μm, typically between 2.0 and2.5 μm.

The EL display device shown in FIG. 1 is characterized in that the LDDregion 33 is formed between the drain region 32 and the channel formingregion 34 in the current control TFT 202. In addition, the LDD region 33has both a region in which the LDD region 33 is overlapped with the gateelectrode 35 with a gate insulating film 18 interposed therebetween, anda region in which the LDD region 33 is not overlapped with the gateelectrode 35 with a gate insulating film 18 interposed therebetween.

The current control TFT 202 supplies a current for allowing the ELelement 203 to emit light, and at the same time controls the amount ofcurrent supplied and enables gray scale display. It is thereforenecessary to prevent deterioration due to hot carrier injection whichoccurs when the current flows. Furthermore, when black is displayed, thecurrent control TFT 202 is set in the off state, but if the off currentvalue is high, then a clean black color display becomes impossible, andthis invites problems such as a reduction in contrast. It is thereforenecessary to suppress the value of the off current.

Regarding deterioration due to hot carrier injection, it is known that astructure in which the LDD region overlaps the gate electrode isextremely effective. However, if the entire LDD region is made tooverlap the gate electrode, the value of the off current rises, andtherefore the applicant of the present invention has proposed both thehot carrier and off current value countermeasures at the same time by anovel structure in which an LDD region which does not overlap the gateelectrode is formed in series.

The length of the LDD region which overlaps the gate electrode may bemade from 0.1 to 3 μm (preferable between 0.3 and 1.5 μm) at this point.If it is too long, then the parasitic capacitance will become larger,and if it is too short, then the effect of preventing hot carrier willbecome weakened. Further, the length of the LDD region not overlappingthe gate electrode may be set from 1.0 to 3.5 μm (preferable between 1.5and 2.0 μm). If it is too long, then a sufficient current becomes unableto flow, and if it is too short, then the effect of reducing off currentvalue becomes weakened.

A parasitic capacitance is formed in the above structure in the regionwhere the gate electrode and the LDD region overlap, and therefore it ispreferable that this region not be formed between the source region 31and the channel forming region 34. The carrier (electrons in this case)flow direction is always the same for the current control TFT, andtherefore it is sufficient to form the LDD region on only the drainregion side.

Further, looking from the viewpoint of increasing the amount of currentthat is able to flow, it is effective to make the film thickness of theactive layer (especially the channel forming region) of the currentcontrol TFT 202 thick (preferably from 50 to 100 nm, more preferablybetween 60 and 80 nm). Conversely, looking from the point of view ofmaking the off current value smaller for the switching TFT 201, it iseffective to make the film thickness of the active layer (especially thechannel forming region) thin (preferably from 20 to 50 nm, morepreferably between 25 and 40 nm).

Next, reference numeral 41 denotes a first passivation film, and itsfilm thickness may be set from 10 nm to 1 μm (preferably between 200 and500 nm). An insulating film containing silicon (in particular,preferably a silicon oxynitride film or a silicon nitride film) can beused as the passivation film material. The passivation film 41 plays therole of protecting the manufactured TFT from contaminant matter andmoisture. Alkaline metals such as sodium are contained in an EL layerformed on the final TFT. In other words, the first passivation film 41works as a protecting layer so that these alkaline metals (mobile ions)do not penetrate into the TFT. Note that alkaline metals andalkaline-earth metals are contained in the term ‘alkaline metal’throughout this specification.

Further, by making the passivation film 41 possess a heat radiationeffect, it is also effective in preventing thermal degradation of the ELlayer. Note that light is emitted from the base 11 side in the FIG. 1structure of the EL display device, and therefore it is necessary forthe passivation film 41 to have light transmitting characteristics.

A chemical compound containing at least one element selected from thegroup consisting of B (boron), C (carbon), and N (nitrogen), and atleast one element selected from the group consisting of Al (aluminum),Si (silicon), and P (phosphorous) can be given as a light transparentmaterial possessing heat radiation qualities. For example, it ispossible to use: an aluminum nitride compound, typically aluminumnitride (Al_(x)N_(y)); a silicon carbide compound, typically siliconcarbide (Si_(x)C_(y)); a silicon nitride compound, typically siliconnitride (Si_(x)N_(y)); a boron nitride compound, typically boron nitride(B_(x)N_(y)); or a boron phosphate compound, typically boron phosphate(B_(x)P_(y)). Further, an aluminum oxide compound, typically aluminumoxide (Al_(x)O_(y)), has superior light transparency characteristics,and has a thermal conductivity of 20 Wm⁻K⁻¹, and can be said to be apreferable material. These materials not only possess heat radiationqualities, but also are effective in preventing the penetration ofsubstances such as moisture and alkaline metals. Note that x and y arearbitrary integers for the above transparent materials.

The above chemical compounds can also be combined with another element.For example, it is possible to use nitrated aluminum oxide, denoted byAlN_(x)O_(y), in which nitrogen is added to aluminum oxide. Thismaterial also not only possesses heat radiation qualities, but also iseffective in preventing the penetration of substances such as moistureand alkaline metals. Note that x and y are arbitrary integers for theabove nitrated aluminum oxide.

Furthermore, the materials recorded in Japanese Patent ApplicationLaid-open No. Sho 62-90260 can also be used. Namely, a chemical compoundcontaining Si, Al, N, O, and M can also be used (note that M is arare-earth element, preferably an element selected from the groupconsisting of Ce (cesium), Yb (ytterbium), Sm (samarium), Er (erbium), Y(yttrium), La (lanthanum), Gd (gadolinium), Dy (dysprosium), and Nd(neodymium)). These materials not only possess heat radiation qualities,but also are effective in preventing the penetration of substances suchas moisture and alkaline metals.

Furthermore, carbon films such as a diamond thin film or amorphouscarbons (especially those which have characteristics close to those ofdiamond; referred to as diamond-like carbon) can also be used. Thesehave very high thermal conductivities, and are extremely effective asradiation layers. Note that if the film thickness becomes larger, thesematerials become brown, and the transmissivity is reduced, and thereforeit is preferable to use them with a film thickness (preferably between 5and 100 nm) as thin as possible.

Note that the aim of the first passivation film 41 is in protecting theTFT from contaminating impurity and from moisture, and therefore it mustbe prepared so as to not lose this effect. A thin film made from amaterial possessing the above radiation effect can be used by itself,but it is effective to laminate this thin film and a thin film havingshielding properties against alkaline metals and moisture (typically asilicon nitride film (Si_(x)N_(y)) or a silicon oxynitride film(SiO_(x)N_(y))). Note that x and y are arbitrary integers for the abovesilicon nitride films and silicon oxynitride films.

Reference numeral 42 denotes a color filter, and reference numeral 43denotes a fluorescent substance (also referred to as a fluorescentpigment layer). Both are a combination of the same color, and containred (R), green (G), or blue (B). The color filter 42 is formed in orderto increase the color purity, and the fluorescent substance 43 is formedin order to perform color transformation.

Note that the method for color display of the EL display devices isroughly divided into four types of color displays: a method of formingthree types of EL elements corresponding to R, G, and B; a method ofcombining white color emissive EL elements with color filters; a methodof combining blue or blue-green emissive EL elements and fluorescentsubstance (fluorescing color change layer, CCM); and a method of using atransparent electrode as a cathode (opposing electrode) and overlappingEL elements corresponding to R, G, and B.

The structure of FIG. 1 is an example of a case of using a combinationof blue emissive EL elements and a fluorescent substance. A blue coloremitting luminescence layer is used as the EL element 203 here, lightpossessing blue color region wavelength, including ultraviolet light, isemitted and the fluorescent substance 43 is excited by the light to emitred, green, or blue light. The color purity of the light is increased bythe color filter 42, and this is outputted.

Note that it is possible to implement the present invention withoutbeing concerned with the method of luminescence, and that all four ofthe above methods can be used with the present invention.

Furthermore, after forming the color filter 42 and the fluorescentsubstance 43, leveling is performed by a second interlayer insulatingfilm 44. A resin film is preferable as the second interlayer insulatingfilm 44, and one such as polyimide, polyamide, acrylic, or BCB(benzocyclobutane) may be used. An inorganic film may, of course, alsobe used, provided that sufficient leveling is possible.

The leveling of steps due to the TFT by the second interlayer insulatingfilm 44 is extremely important. The EL layer formed afterward is verythin, and therefore there are cases in which poor luminescence is causedby the existence of a step. It is therefore preferable to performleveling before forming a pixel electrode so as to be able to form theEL layer on as level a surface as possible.

Furthermore, it is effective to form an insulating film having a highthermal radiation effect (hereafter referred to as a thermal radiationlayer) on the second interlayer insulating film 44. A film thickness of5 nm to 1 μm (typically between 20 and 300 nm) is preferable. This typeof thermal radiation layer functions so that the heat generated by theEL element is released, so that heat is not stored in the EL element.Further, when formed by a resin film, the second interlayer insulatingfilm 44 is weak with respect to heat, and the thermal radiation layerworks so as not to impart bad influence due to the heat generated by theEL element.

It is effective to perform leveling of the TFT by the resin film inmanufacturing the EL display device, as stated above, but there has notbeen a conventional structure which considers the deterioration of theresin film due to heat generated by the EL element. It can therefore besaid that the formation of the thermal radiation layer is extremelyeffective in resolving this point.

Furthermore, provided that a material which is not permeable tomoisture, oxygen, or alkaline metals (a material similar to that of thefirst passivation film 41) is used as the thermal radiation layer, thenit can also function as a protecting layer in order that alkaline metalswithin the EL layer do not diffuse toward the TFT, at the same time aspreventing deterioration of the EL element and the resin film due toheat, as above. In addition, the thermal radiation layer also functionsas a protecting layer so that moisture and oxygen do not penetrate intothe EL layer from the TFT.

In particular, provided that the thermal radiation effect is desired, acarbon film such as a diamond film or a diamond-like carbon film ispreferable, and in order to prevent penetration of substances such asmoisture, it is more preferable to use a lamination structure of acarbon film and a silicon nitride film (or a silicon oxynitride film).

A structure in which TFT side and EL element side are segregated by aninsulating film which has a high radiation effect and is capable ofshielding moisture and alkaline metal, is thus effective.

Reference numeral 45 denotes a pixel electrode (EL element anode) madefrom a transparent conducting film. After opening a contact hole in thesecond interlayer insulating film 44 and in the first passivation film41, the pixel electrode 45 is formed so as to be connected to the drainwiring 37 of the current control TFT 202.

An EL layer (an organic material is preferable) 46, a cathode 47, and aprotecting electrode 48 are formed in order on the pixel electrode 45. Asingle layer structure or a lamination structure can be used as the ELlayer 46, but there are many cases in which the lamination structure isused. Various lamination structures have been proposed, such as thecombinations of layers such as a luminescence layer, an electrontransporting layer, an electron injecting layer, a hole injecting layer,and a hole transporting layer, but any structure may be used for thepresent invention. Doping of a fluorescent pigment into the EL layer mayalso be performed, of course. Note that a light emitting element formedby a pixel electrode (anode), an EL layer, and a cathode is referred toas an EL element throughout this specification.

All already known EL materials can be used by the present invention.Organic materials are widely known as such materials, and consideringthe driver voltage, it is preferable to use an organic material. Forexample, the materials disclosed in the following U.S. patents andJapanese patent applications can be used as the organic EL material:

U.S. Pat. No. 4,356,429; U.S. Pat. No. 4,539,507; U.S. Pat. No.4,720,432; U.S. Pat. No. 4,769,292; U.S. Pat. No. 4,885,211; U.S. Pat.No. 4,950,950; U.S. Pat. No. 5,059,861; U.S. Pat. No. 5,047,687; U.S.Pat. No. 5,073,446; U.S. Pat. No. 5,059,862; U.S. Pat. No. 5,061,617;U.S. Pat. No. 5,151,629; U.S. Pat. No. 5,294,869; U.S. Pat. No.5,294,870; Japanese Patent Application Laid-open No. Hei 10-189525;Japanese Patent Application Laid-open No. Hei 8-241048; and JapanesePatent Application Laid-open No. Hei 8-78159.

Specifically, an organic material such as the one shown by the followinggeneral formula can be used as a hole injecting layer.

Here, Q is either N or a C—R (carbon chain); M is a metal, a metaloxide, or a metal halide; R is hydrogen, an alkyl, an aralkyl, an aryl,or an alkylaryl; and T1 and T2 are unsaturated six member ringsincluding substituent such as hydrogen, alkyl, or halogen.

Furthermore, an aromatic tertiary amine can be used as an organicmaterial hole transporting layer, preferably including thetetraaryldiamine shown by the following general formula.

In formula 2 Are is an arylene group, n is an integer from 1 to 4, andAr, R₇, R₈, and R₉ are each various chosen aryl groups.

In addition, a metal oxynoid compound can be used as an organic materialEL layer, electron transporting layer, or electron injecting layer. Amaterial such as that shown by the general formula below may be used asthe metal oxinoid compound.

It is possible to substitute R₂ through R₇, and a metal oxinoid such asthe following can also be used.

In formula 4, R₂ through R₇ are defined as stated above; L₁ through L₅are carbohydrate groups containing from 1 to 12 carbon elements; andboth L₁ and L₂, or both L₂ and L₃ are formed by benzo-rings. Further, ametal oxinoid such as the following may also be used.

It is possible to substitute R₂ through R₆ here. Coordination compoundshaving organic ligands are thus included as organic EL materials. Notethat the above examples are only some examples of organic EL materialswhich can be used as the EL material of the present invention, and thatthere is absolutely no need to limit the EL material to these.

Furthermore, when using an ink jet method for forming the EL layer, itis preferable to use a polymer material as the EL material. Polymermaterials such as the following can be given as typical polymermaterials: polyparaphenylenevinylenes (PPVs); and polyfluorenes. Forcolorization, it is preferable to use, for example, acyano-polyparaphenylenevinylene in a red emissive material; apolyphenylenevinylene in a green emissive material; and apolyphenylenevinylene and a polyalkylphenylene in a blue emissivematerial. Regarding organic EL materials which can be used in an ink-jetmethod, all of the materials recorded in Japanese Patent ApplicationLaid-open No. Hei 10-012377 can be cited.

Furthermore, a material containing a low work function material such asmagnesium (Mg), lithium (Li), cesium (Cs), barium (Ba), potassium (K),beryllium (Be), or calcium (Ca) is used as the cathode 47. Preferably,an electrode made from MgAg (a material made from Mg and Ag at a mixtureratio of Mg:Ag=10:1) may be used. In addition, a MgAgAl electrode, aLiAl electrode, and a LiFAl electrode can be given as other examples.Further, the protecting electrode 48 is an electrode formed in order toprotect the cathode 47 against moisture from external, and a materialcontaining aluminum (Al) or silver (Ag) is used. The protectingelectrode 48 also has a heat radiation effect.

Note that it is desirable to form the EL layer 46 and the cathode 47 insuccession, without exposure to the atmosphere. In other words, nomatter what type of lamination structure the EL layer and the cathodecontain, it is preferable to form everything in a multi-chamber (alsoreferred to as a cluster tool) type deposition device in succession.This is in order to avoid the absorption of moisture when the EL layeris exposed to the atmosphere because if an organic material is used asthe EL layer, then it is extremely weak with respect to moisture. Inaddition, not only the EL layer 46 and the cathode 47, it is even betterto form all the way through the protecting electrode 48 in succession.

The EL layer is extremely weak with respect to heat, and therefore it ispreferable to use vacuum evaporation (in particular, an organicmolecular beam evaporation method is effective in that it forms a verythin film, on the molecular order level), sputtering, plasma CVD, spincoating, screen printing, or ion plating as the film deposition method.It is also possible to form the EL layer by an ink-jet method. For theink jet method there is a bubble jet method using cavitation (refer toJapanese Patent Application Laid-open No. Hei 5-116297), and there is apiezo method using a piezo element (refer to Japanese Patent ApplicationLaid-open No. Hei 8-290647), and in view of the fact that organic ELmaterials are weak with respect to heat, the piezo method is preferable.

Reference numeral 49 denotes a second passivation film, and its filmthickness may be set from 10 nm to 1 μm (preferable between 200 and 500nm). The object of forming the second passivation film 49 is mainly toprotect the EL layer 46 from moisture, but the second passivation film49 may possess a heat radiation effect, similar to the first passivationfilm 41. The same materials as used for the first passivation film 41can therefore be used as the formation material of the secondpassivation film 49. Note that when an organic material is used as theEL layer 46, it deteriorates due to bonding with oxygen, and thereforeit is preferable to use an insulating film which does not easily releaseoxygen.

Further, the EL layer is weak with respect to heat, as stated above, andtherefore it is preferable to perform film deposition at a lowtemperature as possible (preferably in the range from room temperatureto 120° C.). It can therefore be said that plasma CVD, sputtering,vacuum evaporation, ion plating, and solution application (spin coating)are desirable film deposition methods.

The EL display device of the present invention has a pixel portioncontaining a pixel with a structure as stated above, and TFTs havingdiffering structures in response to their function are arranged in thepixel. A switching TFT having a sufficiently low off current value, anda current control TFT which is strong with respect to hot carrierinjection can be formed within the same pixel, and an EL display devicehaving high reliability and which is capable of good image display canthus be formed.

Note that the most important point in the pixel structure of FIG. 1 isthat a multi-gate structure TFT is used as the switching TFT, and thatit is not necessary to place limits on the structure of FIG. 1 withregard to the constitution such as the arrangement of LDD regions.

A more detailed explanation of the present invention, having the aboveconstitution, is now performed by the embodiments shown below.

Embodiment 1

The embodiments of the present invention are explained using FIGS. 3A to5C. A method of manufacturing a pixel portion, and TFTs of a drivercircuit portion formed in the periphery of the pixel portion, isexplained here. Note that in order to simplify the explanation, a CMOScircuit is shown as a basic circuit for the driver circuits.

First, as shown in FIG. 3A, a base film 301 is formed with a 300 nmthickness on a glass substrate 300. Silicon oxynitride films arelaminated as the base film 301 in embodiment 1. It is preferred to setthe nitrogen concentration to between 10 and 25 wt % in the region ofthe film contacting the glass substrate 300.

Further, it is effective to form a heat radiating layer, made from thesame material as that of the first passivation film 41 shown in FIG. 1,as a portion of the base film 301. A large electric current flows in acurrent control TFT, heat is easily generated, and therefore it iseffective to form the heat radiating layer as close as possible to thecurrent control TFT.

Next, an amorphous silicon film (not shown in the figures) is formedwith a thickness of 50 nm on the base film 301 by a known depositionmethod. Note that it is not necessary to limit this to the amorphoussilicon film, and another film may be formed provided that it is asemiconductor film containing an amorphous structure (including amicrocrystalline semiconductor film). In addition, a compoundsemiconductor film containing an amorphous structure, such as anamorphous silicon germanium film, may also be used. Further, the filmthickness may be made from 20 to 100 nm.

The amorphous silicon film is then crystallized by a known method,forming a crystalline silicon film (also referred to as apolycrystalline silicon film or a polysilicon film) 302. Thermalcrystallization using an electric furnace, laser annealingcrystallization using a laser, and lamp annealing crystallization usingan infrared lamp exist as known crystallization methods. Crystallizationis performed in embodiment 1 using light from an excimer laser whichuses XeCl gas.

Note that pulse emission type excimer laser light formed into a linearshape is used in embodiment 1, but a rectangular shape may also be used,and continuous emission argon laser light and continuous emissionexcimer laser light can also be used.

The crystalline silicon film is used as an active layer of the TFTs inembodiment 1, but it is also possible to use an amorphous silicon filmas the active layer. However, it is necessary for a large current toflow through the current control TFT, and therefore it is more effectiveto use the crystalline silicon film, through which current easily flows.

Note that it is effective to form the active layer of the switching TFT,in which there is a necessity to reduce the off current, by theamorphous silicon film, and to form the active layer of the currentcontrol TFT by the crystalline silicon film. Electric current flows withdifficulty in the amorphous silicon film because the carrier mobility islow, and the off current does not easily flow. In other words, the mostcan be made of the advantages of both the amorphous silicon film,through which current does not flow easily, and the crystalline siliconfilm, through which current easily flows.

Next, as shown in FIG. 3B, a protecting film 303, is formed on thecrystalline silicon film 302 from a silicon oxide film having athickness of 130 nm. This thickness may be chosen within the range of100 to 200 nm (preferably between 130 and 170 nm). Furthermore, otherfilms may also be used providing that they are insulating filmscontaining silicon. The protecting film 303 is formed so that thecrystalline silicon film is not directly exposed to plasma duringaddition of an impurity, and so that it is possible to have delicateconcentration control of the impurity.

Resist masks 304 a and 304 b are then formed on the protecting film 303,and an impurity element which imparts n-type conductivity (hereafterreferred to as an n-type impurity element) is added. Note that elementsresiding in periodic table group 15 are generally used as the n-typeimpurity element, and typically phosphorous or arsenic can be used. Notethat a plasma doping method is used, in which phosphine (PH₃) is plasmaactivated without separation of mass, and phosphorous is added at aconcentration of 1×10¹⁸ atoms/cm³ in embodiment 1. An ion implantationmethod, in which separation of mass is performed, may also be used, ofcourse.

The dose amount is regulated so that the n-type impurity element iscontained in n-type impurity regions 305 and 306, thus formed by thisprocess, at a concentration of 2×10¹⁶ to 5×10⁹ atoms/cm³ (typicallybetween 5×10¹⁷ and 5×10¹⁸ atoms/cm³).

Next, as shown in FIG. 3C, the protecting film 303 is removed, andactivation of the added periodic table group 15 element is performed. Aknown technique of activation may be used as the means of activation,and activation is done in embodiment 1 by irradiation of excimer laserlight. Both of pulse emission type laser and a continuous emission typelaser may be used, and it is not necessary to place any limits on theuse of excimer laser light. The goal is the activation of the addedimpurity element, and it is preferable that irradiation is performed atan energy level at which the crystalline silicon film does not melt.Note that the laser irradiation may also be performed with theprotecting film 303 in place.

Activation by heat treatment may also be performed along with activationof the impurity element by laser light. When activation is performed byheat treatment, considering the heat resistance of the substrate, it isgood to perform heat treatment on the order of 450 to 550° C.

A boundary portion (connecting portion) with regions along the edges ofthe n-type impurity regions 305 and 306, namely regions along theperimeter into which the n-type impurity element, which exists in then-type impurity regions 305 and 306, is not added, is delineated by thisprocess. This means that, at the point when the TFTs are latercompleted, extremely good connections can be formed between LDD regionsand channel forming regions.

Unnecessary portions of the crystalline silicon film are removed next,as shown in FIG. 3D, and island shape semiconductor films (hereafterreferred to as active layers) 307 to 310 are formed.

Then, as shown in FIG. 3E, a gate insulating film 311 is formed,covering the active layers 307 to 310. An insulating film containingsilicon and with a thickness of 10 to 200 nm, preferably between 50 and150 nm, may be used as the gate insulating film 311. A single layerstructure or a lamination structure may be used. A 110 nm thick siliconoxynitride film is used in embodiment 1.

A conducting film with a thickness of 200 to 400 nm is formed next andpatterned, forming gate electrodes 312 to 316. Note that in embodiment1, the gate electrodes and lead wirings electrically connected to thegate electrodes (hereafter referred to as gate wirings) are formed fromdifferent materials. Specifically a material having a lower resistancethan that of the gate electrodes is used for the gate wirings. This isbecause a material which is capable of being micro-processed is used asthe gate electrodes, and even if the gate wirings cannot bemicro-processed, the material used for the wirings has low resistance.Of course, the gate electrodes and the gate wirings may also be formedfrom the same material.

Further, the gate wirings may be formed by a single layer conductingfilm, and when necessary, it is preferable to use a two layer or a threelayer lamination film. All known conducting films can be used as thegate electrode material. However, as stated above, it is preferable touse a material which is capable of being micro-processed, specifically,a material which is capable of being patterned to a line width of 2 μmor less.

Typically, a film of a material chosen from among the group consistingof tantalum (Ta), titanium (Ti), molybdenum (Mo), tungsten (W), andchromium (Cr); or a nitrated compound of the above elements (typically atantalum nitride film, a tungsten nitride film, or a titanium nitridefilm); or an alloy film of a combination of the above elements(typically a Mo—W alloy or a Mo—Ta alloy); or a silicide film of theabove elements (typically a tungsten silicide film or a titaniumsilicide film); or a silicon film which has been made to possessconductivity can be used. A single layer film or a lamination may beused, of course.

A lamination film made from a 50 nm thick tantalum nitride (TaN) filmand a 350 nm thick Ta film is used in embodiment 1. It is good to formthis film by sputtering. Furthermore, if an inert gas such as Xe or Neis added as a sputtering gas, then film peeling due to the stress can beprevented.

The gate electrodes 313 and 316 are formed at this time so as to overlapa portion of the n-type impurity regions 305 and 306, respectively,sandwiching the gate insulating film 311. This overlapping portion laterbecomes an LDD region overlapping the gate electrode.

Next, an n-type impurity element (phosphorous is used in embodiment 1)is added in a self-aligning manner with the gate electrodes 312 to 316as masks, as shown in FIG. 4A. The addition is regulated so thatphosphorous is added to impurity regions 317 to 323 thus formed at aconcentration of 1/10 to ½ that of the impurity regions 305 and 306(typically between ¼ and ⅓). Specifically, a concentration of 1×10¹⁶ to5×10¹⁸ atoms/cm³ (typically 3×10¹⁷ to 3×10¹³ atoms/cm³) is preferable.

Resist masks 324 a to 324 d are formed next to cover the gateelectrodes, as shown in FIG. 4B, and an n-type impurity element(phosphorous is used in embodiment 1) is added, forming impurity regions325 to 331 containing a high concentration of phosphorous. Ion dopingusing phosphine (PH₃) is also performed here, and is regulated so thatthe phosphorous concentration of these regions is from 1×10²⁰ to 1×10²¹atoms/cm³ (typically between 2×10²⁰ and 5×10²⁰ atoms/cm³).

A source region or a drain region of the n-channel TFT is formed by thisprocess, and in the switching TFT, a portion of the n-type impurityregions 320 to 322 formed by the process of FIG. 4A remains. Theseremaining regions correspond to the LDD regions 15 a to 15 d of theswitching TFT in FIG. 1.

Next, as shown in FIG. 4C, the resist masks 324 a to 324 d are removed,and a new resist mask 332 is formed. A p-type impurity element (boron isused in embodiment 1) is then added, forming impurity regions 333 and334 containing a high concentration of boron. Boron is added here to aconcentration of 3×10²° to 3×10²¹ atoms/cm³ (typically between 5×10²⁰and 1×10²¹ atoms/cm³) by ion doping using diborane (B₂H₆).

Note that phosphorous has already been added to the impurity regions 333and 334 at a concentration of 1×10¹⁶ to 5×10¹⁸ atoms/cm³, but boron isadded here at a concentration of at least 3 times that of thephosphorous. Therefore, the n-type impurity regions already formedcompletely invert to p-type, and function as p-type impurity regions.

Next, after removing the resist mask 332, the n-type and p-type impurityelements added at various concentrations are activated. Furnaceannealing, laser annealing, or lamp annealing may be performed as ameans of activation. Heat treatment is performed in embodiment 1 in anitrogen atmosphere for 4 hours at 550° C. in an electric furnace.

It is important to remove as much of the oxygen in the atmosphere aspossible at this time. This is because if any oxygen exists, then theexposed surface of the electrode is oxidized, inviting an increase inresistance, and at the same time it becomes more difficult to later makean ohmic contact. It is therefore preferable that the concentration ofoxygen in the atmosphere in the above activation process be 1 ppm orless, desirably 0.1 ppm or less.

After the activation process is completed, a gate wiring 335 with athickness of 300 nm is formed next. A metallic film having aluminum (Al)or copper (Cu) as its principal constituent (comprising 50 to 100% ofthe composition) may be used as the material of the gate wiring 335. Aswith the gate wiring 211 of FIG. 2, the gate wiring 335 is formed with aplacement so that the gate electrodes 314 and 315 of the switching TFTs(corresponding to gate electrodes 19 a and 19 b of FIG. 2) areelectrically connected. (See FIG. 4D.)

The wiring resistance of the gate wiring can be made extremely small byusing this type of structure, and therefore a pixel display region(pixel portion) having a large surface area can be formed. Namely, thepixel structure of embodiment 1 is extremely effective because an ELdisplay device having a screen size of a 10 inch diagonal or larger (inaddition, a 30 inch or larger diagonal) is realized.

A first interlayer insulating film 336 is formed next, as shown in FIG.5A. A single layer insulating film containing silicon is used as thefirst interlayer insulating film 336, but a lamination film may becombined in between. Further, a film thickness of between 400 nm and 1.5.mu.m may be used. A lamination structure of an 800 nm thick siliconoxide film on a 200 nm thick silicon oxynitride film is used inembodiment 1.

In addition, heat treatment is performed for 1 to 12 hours at 300 to450° C. in an atmosphere containing between 3 and 100% hydrogen,performing hydrogenation. This process is one of hydrogen termination ofdangling bonds in the semiconductor film by hydrogen which is thermallyactivated. Plasma hydrogenation (using hydrogen activated by a plasma)may also be performed as another means of hydrogenation.

Note that the hydrogenation step may also be inserted during theformation of the first interlayer insulating film 336. Namely, hydrogenprocessing may be performed as above after forming the 200 nm thicksilicon oxynitride film, and then the remaining 800 nm thick siliconoxide film may be formed.

A contact hole is formed next in the first interlayer insulating film336, and source wirings 337 to 340, and drain wirings 341 to 343 areformed. In embodiment 1, a lamination film with a three layer structureof a 100 nm titanium film, a 300 nm aluminum film containing titanium,and a 150 nm titanium film, formed successively by sputtering, is usedas these wirings. Other conducting films may also be used, of course,and an alloy film containing silver, palladium, and copper may also beused.

A first passivation film 344 is formed next with a thickness of 50 to500 nm (typically between 200 and 300 nm). A 300 nm thick siliconoxynitride film is used as the first passivation film 344 inembodiment 1. This may also be substituted by a silicon nitride film. Itis of course possible to use the same materials as those of the firstpassivation film 41 of FIG. 1.

The respective film thickness may be chosen in the range of 0.5 to 5 μm(typically between 1 and 2 μm). In particular, the optimal filmthickness of the fluorescing body 346 varies with the material used. Inother words, if it is too thin, then the color transformation efficiencybecomes poor, and if it is too thick, then the step becomes large andthe amount of light transmitted drops. Optimal film thicknesses musttherefore be set by taking a balance of both characteristics.

Note that, in embodiment 1, an example of a color changing method inwhich the light emitted from the EL layer is converted in color, but ifa method of manufacturing individual EL layers which correspond to R, G,and B, is employed, then the color filter and the fluorescing body canbe omitted.

A second interlayer insulating film 347 is formed next from an organicresin. Materials such as polyimide, polyamide, acrylic, and BCB(benzocyclobutene) can be used as the organic resin. In particular, themain purpose of the second interlayer insulating film 347 is to levelthe step, and therefore acrylic, having superior levelingcharacteristics, is preferable. An acrylic film is formed in embodiment1 with a film thickness which can sufficiently level the step betweenthe color filter 345 and the fluorescing body 346. This thickness ispreferably from 1 to 5 μm (more preferably between 2 and 4 μm).

A contact hole for reaching the drain wiring 343 is formed next in thesecond inter layer insulating film 347 and in the first passivation film344, and a pixel electrode 348 is formed. A compound of indium oxide andtin oxide is formed into 110 nm thick in embodiment 1, and patterning isperformed, making the pixel electrode. The pixel electrode 348 becomesan anode of the EL element. Note that it is also possible to use othermaterials: a compound film of indium oxide and zinc oxide, or a zincoxide film containing gallium oxide.

Note that embodiment 1 becomes a structure in which the pixel electrode348 is electrically connected to the drain region 331 of the currentcontrol TFT, through the drain wiring 343. This structure has thefollowing advantages.

The pixel electrode 348 becomes directly connected to an organicmaterial such as the EL layer (emitting layer) or a charge transportinglayer, and therefore it is possible for the mobile ions contained in theEL layer to diffuse throughout the pixel electrode. In other words,without connecting the pixel electrode 348 directly to the drain region331, a portion of the active layer, the introduction of mobile ions intothe active layer due to the drain wiring 343 being interrupted can beprevented in the structure of embodiment 1.

Next, as shown in FIG. 5C, an EL layer 349, a cathode (MgAg electrode)350, and a protecting electrode 351 are formed in succession withoutexposure to the atmosphere. It is preferable, at this point, to performheat treatment of the pixel electrode 348, completely removing allmoisture, before forming the EL layer 349 and the cathode 350. Note thata known material can be used as the EL layer 349.

The materials explained in the “embodiment mode” section of thisspecification can be used as the EL layer 349. In embodiment 1, an ELlayer having a 4 layer structure of a hole injecting layer, a holetransporting layer, an emitting layer, and an electron transportinglayer is used, as shown in FIG. 19, but there are cases in which theelectron transporting layer is not formed, and cases in which anelectron injecting layer is also formed. Furthermore, there are alsocases in which the hole injecting layer is omitted. Several examples ofthese types of combinations have already been reported, and any of theseconstitutions may be used.

An amine such as TPD (triphenylamine dielectric) may be used as the holeinjecting layer or as the hole transporting layer, and in addition, ahydrazone (typically DEH), a stilbene (typically STB), or a starburst(typically m-MTDATA) can also be used. In particular, a starburstmaterial, which has a high glass transition temperature and is difficultto crystallize, is preferable. Further, polyaniline (PAni),polythiophene (PEDOT), and copper phthalocyanine (CuPc) may also beused.

BPPC, perylene, and DCM can be used as a red color emitting layer in theemitting layer, and in particular, the Eu complex shown byEu(DBM)₃(Phen) (refer to Kido, J., et. al, Appl. Phys., vol. 35, pp.L394-6, 1996 for details) is highly monochromatic, possessing a sharpemission at a wavelength of 620 nm.

Further, typically an Alg₃ (8-hydroxyquinoline aluminum) material inwhich quinacridone or coumarin is added at a level of several mol % canbe used as a green color emitting layer.

The chemical formula is as shown below.

-   -   [formula 6]

In addition, typically a distyryl-arylene amino derivative, in whichamino substituted DSA is added to DSA (distyryl-arylene derivative) canbe used as a blue color emitting layer. In particular, it is preferableto use the high performance material distyryl-biphenyl (DPVBi). Itschemical formula is as shown below.

-   -   [formula 7]

Further, a 300 nm thick silicon nitride film is formed as a secondpassivation film 352, and this may also be formed in succession, withoutexposure to the atmosphere, after formation of the protecting electrode351. The same materials as those of the second passivation film 49 ofFIG. 1 can also be used, of course, as the second passivation film 352.

A 4 layer structure made from a hole injecting layer, a holetransporting layer, an emitting layer, and an electron injecting layeris used in embodiment 1, but there are already examples of manycombinations already reported, and any of these constitutions may alsobe used. Furthermore, an MgAg electrode is used as the cathode of the ELelement in embodiment 1, but other known materials may also be used.

The protecting electrode 351 is formed in order to prevent deteriorationof the MgAg electrode 350, and a metallic film having aluminum as itsprincipal constituent is typical. Other materials may, of course, alsobe used. Furthermore, the EL layer 349 and the MgAg electrode 350 areextremely weak with respect to moisture, and therefore it is preferableto perform successive formation up through to the protecting electrode351 without exposure to the atmosphere, protecting the EL layer fromexternal air.

Note that the film thickness of the EL layer 349 may be from 10 to 400nm (typically between 60 and 160 nm), and that the thickness of the MgAgelectrode 350 may be from 180 to 300 nm (typically between 200 and 250nm).

The active matrix type EL display device with the structure shown inFIG. 5C is thus completed. By arranging TFTs with optimal structure innot only the pixel portion, but also in the driver circuit portion, theactive matrix type EL display device of embodiment 1 shows extremelyhigh reliability, and the operational characteristics can be raised.

First, a TFT having a structure which reduces hot carrier injection asmuch as possible without a drop in the operation speed is used as ann-channel TFT 205 of the CMOS circuit forming the driver circuits. Notethat the driver circuits referred to here include circuits such as ashift register, a buffer, a level shifter, and a sampling circuit (alsoreferred to as a transfer gate). When digital driving is performed,signal conversion circuits such as a D/A converter circuit are alsoincluded.

In the case of embodiment 1, an active layer of the n-channel TFT 205includes a source region 355, a drain region 356, an LDD region 357, anda channel forming region 358, as shown in FIG. 5C, and the LDD region357 overlaps the gate electrode 313, sandwiching the gate insulatingfilm 311.

The formation of the LDD region on the drain side only is inconsideration of not lowering the operation speed. Further, it is notnecessary to be concerned with the value of the off current in then-channel TFT 205, and greater emphasis may be placed on the operationspeed. It is therefore preferable that the LDD region 357 completelyoverlap the gate electrode 313, reducing resistive components as much aspossible. In other words, it is good to eliminate all offset.

Deterioration of a p-channel TFT 206 of the CMOS circuit due to hotcarrier injection is almost of no concern, and in particular, therefore,an LDD region is not formed. It is also possible, of course, to takeaction against hot carriers by forming an LDD region similar to that ofthe n-channel TFT 205.

Note that among the driver circuits, the sampling circuit is somewhatspecial when compared to the other circuits, and a large current flowsin the channel forming region in both directions. Namely, the roles ofthe source region and the drain region change. In addition, it isnecessary to suppress the value of the off current as much as possible,and with that in mind, it is preferable to arrange a TFT havingfunctions at an intermediate level between the switching TFT and thecurrent control TFT.

It is preferable, therefore, to arrange a TFT with the structure shownin FIG. 9 as an n-type TFT forming the sampling circuit. As shown inFIG. 9, a portion of LDD regions 901 a and 901 b overlap a gateelectrode 903, sandwiching a gate insulating film 902. This effect is asstated in the explanation of the current control TFT 202, and the caseof the sampling circuit differs in the point of forming the LDD regions901 a and 901 b with a shape that sandwiches a channel forming region904.

Further, a pixel with the structure shown in FIG. 1 is formed, forming apixel portion. The structures of a switching TFT and a current controlTFT formed within the pixel have already been explained in FIG. 1, andtherefore that explanation is omitted here.

Note that, in practice, it is preferable to additionally performpackaging (sealing) after completing up through FIG. 5C by using ahousing material such as a highly airtight protecting film (such as alaminar film or an ultraviolet hardened resin film) or a ceramic sealingcan, so that there is no exposure to the atmosphere. By making theinside of the housing material an inert environment, and by placing anabsorbing agent (for example, barium oxide) within the housing material,the reliability (life) of the EL layer is increased.

Furthermore, after the airtightness is increased by the packagingprocessing, a connector (a flexible printed circuit, FPC) for connectingbetween output terminals from elements or circuits formed on thesubstrate, and external signal terminals, is attached, completing amanufactured product. The EL display device in this state of being ableto be shipped is referred to as an EL module throughout thisspecification.

The constitution of the active matrix type EL display device ofembodiment 1 is explained here using the perspective view of FIG. 6. Theactive matrix type EL display device of embodiment 1 is formed on aglass substrate 601, and is composed of a pixel portion 602, a gate sidedriving circuit 603, and a source side driving circuit 604. A switchingTFT 605 of the pixel portion is an n-channel TFT, and is placed at theintersection of a gate wiring 606 connected to the gate side drivingcircuit 603, and a source wiring 607 of the source side driving circuit604. Furthermore, the drain of the switching TFT 605 is electricallyconnected to the gate of a current control TFT 608.

In addition, the source of the current control TFT 608 is connected to acurrent supply line 609, and an EL element 610 is electrically connectedto the drain of the current control TFT 608. Provided that the currentcontrol TFT 608 is an n-channel TFT, it is preferable to connect thecathode of the EL element 610 to the drain of the current control TFT608 at this point. Further, if the current control TFT 608 is ap-channel TFT, then it is preferable to connect the anode of the ELelement 610 to the drain of the current control TFT 608.

Input wirings (connection wirings) 612 and 613, and an input wiring 614which is connected to the current supply line 609, are then formed in anexternal input terminal FPC 611 in order to transfer signals to thedriver circuits.

Shown in FIG. 7 is one example of the circuit composition of the ELdisplay device shown in FIG. 6. The EL display device of embodiment 1has a source side driving circuit 701, a gate side driving circuit (A)707, a gate side driving circuit (B) 711, and a pixel portion 706. Notethat, throughout this specification, driver circuit is a generic termwhich includes source side processing circuits and gate side processingcircuits.

The source side driving circuit 701 is provided with a shift register702, a level shifter 703, a buffer 704, and a sampling circuit (transfergate) 705. In addition, the gate side driving circuit (A) 707 isprovided with a shift register 708, a level shifter 709, and a buffer710. The gate side driving circuit (B) 711 has a similar composition.

The driving voltage is from 5 to 16 V (typically 10 V) for the shifterregisters 702 and 708 here, and the structure shown by reference numeral205 of FIG. 5C is suitable for an n-channel TFT used in a CMOS circuitforming the circuits.

Furthermore, the driving voltage becomes high at between 14 and 16 V forthe level shifters 703 and 709, and for the buffers 704 and 710, andsimilar to the shifters, a CMOS circuit containing the n-channel TFT 205of FIG. 5C is suitable. Note that the use of a multi-gate structure,such as a double gate structure or a triple gate structure for the gatewirings, is effective by increasing the reliability of each circuit.

The driving voltage is between 14 and 16 V for the sampling circuit 705,but it is necessary to reduce the value of the off current because thesource region and the drain region invert, and therefore a CMOS circuitcontaining the n-channel TFT 208 of FIG. 9 is suitable.

In addition, the driving voltage of the pixel portion 706 is between 14and 16 V, and a pixel with the structure shown in FIG. 1 is arranged.

Note that the above constitutions can be easily realized bymanufacturing TFTs in accordance with the manufacturing processes shownin FIGS. 3A to 5C. Furthermore, only the constitution of the pixelportion and the driver circuits is shown in embodiment 1, but it is alsopossible to form other logic circuits, in addition to the drivingcircuits, such as a signal divider circuit, a D/A converter circuit, anop-amp circuit, and a γ compensation circuit on the same substrate andin accordance with the manufacturing process of embodiment 1. Inaddition, it is considered that circuits such as a memory portion and amicroprocessor can also be formed on the same substrate.

An explanation of the EL module of embodiment 1, containing the housingmaterial, is made using FIGS. 17A and 17B. Note that, when necessary,the symbols used in FIGS. 6 and 7 are cited.

A pixel portion 1701, a source side driving circuit 1702, and a gateside driving circuit 1703 are formed on a substrate (including a basefilm underneath a TFT) 1700. Various wirings from the respective drivercircuits are connected to external equipment, via the FPC 611, throughthe input wirings 612 to 614.

A housing material 1704 is formed at this point enclosing at least thepixel portion, and preferably the driving circuits and the pixelportion. Note that the housing material 1704 is of an irregular shape inwhich the internal size is larger than the external size of the ELelement, or has a sheet shape, and is fixed to the substrate 1700 by anadhesive 1705 so as to form an airtight space jointly with the substrate1700. At this point, the EL element is in a state of being completelysealed in the above airtight space, and is completely cutoff from theexternal atmosphere. Note that a multiple number of housing materials1704 may be formed.

It is preferable to use an insulating substance such as a glass or apolymer as the housing material 1704. The following can be given asexamples: amorphous glass (such as borosilicate glass or quartz);crystallized glass; ceramic glass; organic resins (such as acrylicresins, styrene resins, polycarbonate resins, and epoxy resins); andsilicone resins. In addition, ceramics may also be used. Furthermore,provided that the adhesive 1705 is an insulating material, it is alsopossible to use a metallic material such as a stainless alloy.

It is possible to use an adhesive such as an epoxy resin or an acrylateresin as the material of the adhesive 1705. In addition, a thermallyhardened resin or a light hardened resin can also be used as theadhesive. Note that it is necessary to use a material through which, asmuch as is possible, oxygen and moisture is not transmitted.

In addition, it is preferable to fill an opening 1706 between thehousing material and the substrate 1700 with an inert gas (such asargon, helium, or nitrogen). There are no limitations on a gas, and itis also possible to use an inert liquid (such as a liquid fluorinatedcarbon, typically perfluoroalkane). The materials such as those used byJapanese Patent Application Laid-open No. Hei 8-78519 may be referred toregarding inert liquids. The space may also be filled with a resin.

It is effective to form drying agent in the opening 1706. Materials suchas those recorded in Japanese Patent Application Laid-open No. Hei9-148066 can be used as the drying agent. Typically, barium oxide may beused. Furthermore, it is effective to form an antioxidizing agent aswell, not just a drying agent.

A plural number of isolated pixels having EL elements are formed in thepixel portion, as shown in FIG. 17B, and all of the pixels have aprotecting electrode 1707 as a common electrode. In embodiment 1 it ispreferable to form the EL layer, the cathode (MgAg electrode), and theprotecting electrode in succession, without exposure to the atmosphere.The EL layer and the cathode are formed using the same mask material,and provided that only the protecting electrode is formed by a separatemask material, then the structure of FIG. 17B can be realized.

The EL layer and the cathode may be formed only in the pixel portion atthis point, and it is not necessary to form them on the drivingcircuits. There is no problem, of course, with forming them on thedriving circuits, but considering the fact that alkaline metals arecontained in the EL layer, it is preferable to not form it over thedriving circuits.

Note that an input wiring 1709 is connected to the protecting electrode1707 in a region shown by reference numeral 1708. The input wiring 1709is a wiring for providing a preset voltage to the protecting electrode1707, and is connected to the FPC 611 through a conducting pastematerial (typically an anisotropic conducting film) 1710.

A manufacturing process for realizing a contact structure in the region1708 is explained here using FIGS. 18A to 18C.

First, the state of FIG. 5A is obtained in accordance with the processesof embodiment 1. At this point the first interlayer insulating film 336and the gate insulating film 311 are removed from the edges of thesubstrate (in the region shown by reference numeral 1708 in FIG. 17B),and the input wiring 1709 is formed on that region. The source wiringsand the drain wirings of FIG. 5A are of course formed at the same time.(See FIG. 18A.)

Next, when etching the second interlayer insulating film 347 and thefirst passivation film 344 in FIG. 5B, a region shown by referencenumeral 1801 is removed, and an open portion 1802 is formed. (See FIG.18B.)

The processes of forming the EL element (pixel electrode, EL layer, andcathode formation processes) in the pixel portion are performed in thisstate. A mask material is used in the region shown in FIGS. 18A to 18Cat this time so that the EL element is not formed in this region. Afterforming the cathode 349, the protecting electrode 350 is formed using aseparate mask material. The protecting electrode 350 and the inputwiring 1709 are thus electrically connected. Further, the secondpassivation film 352 is formed, and the state of FIG. 18C is obtained.

The contact structure of the region shown by reference numeral 1708 inFIG. 17B is thus realized by the above steps. The input wiring 1709 isthen connected to the FPC 611 through the opening between the housingmaterial 1704 and the substrate 1700 (note that this is filled by theadhesive 1705; in other words, it is necessary for the thickness of theadhesive 1705 to be such that it can sufficiently level the step of theinput wiring). Note that an explanation of the input wiring 1709 is madehere, but the other input wirings 612 to 614 are also similarlyconnected to the FPC 611 by passing under the housing material 1704.

Embodiment 2

In embodiment 2, an example of a pixel constitution is shown in FIG. 10which differs from the constitution shown in FIG. 2B.

The two pixels shown in FIG. 2B are arranged with symmetry around thecurrent supply line in embodiment 2. Namely, as shown in FIG. 10, bymaking the current supply line 213 common between the two pixelsneighboring the current supply line, the number of wirings needed can bereduced. Note that it is not necessary to change the structure of theTFTs placed inside the pixels.

If this type of constitution is used, then it becomes possible tomanufacture a very high definition pixel portion, increasing the imagequality.

Note that the constitution of embodiment 2 can easily be realized inaccordance with the manufacturing processes of embodiment 1, and thatthe explanations of embodiment 1 and of FIG. 1 may be referencedregarding points such as the structure of the TFTs.

Embodiment 3

A case of forming a pixel portion having a structure which differs fromthat of FIG. 1 is explained using FIG. 11 in embodiment 3. Note thatprocesses up through the formation of the second interlayer insulatingfilm 44 may be performed in accordance with embodiment 1. Furthermore,the structures of the switching TFT 201 and the current control TFT 202,covered by the second interlayer insulating film 44, are the same asthose of FIG. 1, and their explanation is therefore omitted.

In the case of embodiment 3, a pixel electrode 51, a cathode 52, and anEL layer 53 are formed after forming a contact hole in the secondinterlayer insulating film 44 and the first passivation film 41. Thecathode 52 and the EL layer 53 are formed in succession, withoutexposure to the atmosphere, by vacuum evaporation in embodiment 3, andat that time a red color emitting EL layer, a green color emitting ELlayer, and a blue color emitting layer are formed selectively inseparate pixels by using a mask material. Note that while only one pixelis shown in FIG. 11, pixels with the same structure are formedcorresponding to the colors of red, green, and blue, respectively, andthat color display can be performed by these pixels. A known materialmay be employed for each EL layer color.

A 150 nm thick aluminum alloy film (an aluminum film containing 1 wt %of titanium) is formed as the pixel electrode 51 in embodiment 3.Provided that it is a metallic material, any material may be used as thepixel electrode material, but it is preferable to use a material havinga high reflectivity. Further, a 230 nm thick MgAg electrode is used asthe cathode 52, and the film thickness of the EL layer 53 is 90 nm(including, from the bottom, a 20 nm electron transporting layer, a 40nm emitting layer, and a 30 nm hole transporting layer).

An anode 54 made from a transparent conducting film (an ITO film inembodiment 3) is formed next with a thickness of 110 nm. An EL element209 is thus formed, and if a second passivation film 55 is formed by thesame materials as shown in embodiment 1, then a pixel with the structureshown in FIG. 11 is completed.

When using the structure of embodiment 3, the red, green, or blue lightgenerated by each pixel is irradiated in the opposite direction as thatof the substrate on which the TFTs are formed. For that reason, almostthe entire area inside the pixel, namely the region in which the TFTsare formed, can be used as an effective emitting region. As a result,there is a sharp increase in the effective emitting surface area of thepixel, and the brightness and the contrast ratio (the ratio betweenlight and dark) of the image are increased.

Note that it is possible to freely combine the composition of embodiment3 with the constitutions of any of embodiments 1 and 2.

Embodiment 4

A case of forming a pixel having a structure which differs from that ofFIG. 2 of embodiment 1 is explained in embodiment 4 using FIGS. 12A and12B.

In FIG. 12A, reference numeral 1201 denotes a switching TFT, whichcomprises an active layer 56, a gate electrode 57 a, a gate wiring 57 b,a source wiring 58, and a drain wiring 59. Further, reference numeral1202 denotes a current. control TFT, which comprises an active layer 60,a gate electrode 61, a source wiring 62, and a drain wiring 63. Thesource wiring 62 of the current control TFT 1202 is connected to acurrent supply line 64, and the drain wiring 63 is connected to an ELelement 65. FIG. 12B shows the circuit composition of this pixel.

The point of difference between FIG. 12A and FIG. 2A is the structure ofthe switching TFT. In embodiment 4 the gate electrode 57 a is formedwith a fine line width between 0.1 and 5 μM, and the active layer 56 isformed in such a way that the active layer 56 traverses the gateelectrode. The gate wiring 57 b is formed so as to electrically connectthe gate electrode 57 a of each pixel. A triple gate structure whichdoes not requires much surface area is thus realized.

Other portions are similar to those of FIG. 2A, and the effectiveemitting surface area becomes larger because the surface areaexclusively used by the switching TFT becomes smaller if the structureof embodiment 4 is employed. In other words, the image brightness isincreased. Furthermore, a gate structure in which redundancy isincreased in order to reduce the value of the off current can berealized, and therefore the image quality can be increased even further.

Note that, in the constitution of embodiment 4, the current supply line64 can be made common between neighboring pixels, as in embodiment 2,and that a structure like that of embodiment 3 may also be used.Furthermore, processes of manufacturing may be performed in accordancewith those of embodiment 1.

Embodiment 5

Cases in which a top gate type TFT is used are explained in embodiments1 to 4, and the present invention may also be implemented using a bottomgate type TFT. A case of implementing the present invention by using areverse stagger type TFT is explained in embodiment 5 using FIG. 13.Note that, except for the structure of the TFT, the structure is thesame as that of FIG. 1, and therefore the same symbols as those of FIG.1 are used when necessary.

In FIG. 13, the similar materials as those of FIG. 1 can be used in thesubstrate 11 and in the base film 12. A switching TFT 1301 and a currentcontrol TFT 1302 are then formed on the base film 12.

The switching TFT 1301 comprises: gate electrodes 70 a and 70 b; a gatewiring 71; a gate insulating film 72; a source region 73; a drain region74; LDD regions 75 a to 75 d; a high concentration impurity region 76;channel forming regions 77 a and 77 b; channel protecting films 78 a and78 b; a first interlayer insulating film 79; a source wiring 80; and adrain wiring 81.

Further, the current control TFT 1302 comprises: a gate electrode 82;the gate insulating film 72; a source region 83; a drain region 84; anLDD region 85; a channel forming region 86; a channel protecting film87; a first interlayer insulating film 79; a source wiring 88; and adrain wiring 89. The gate electrode 82 is electrically connected to thedrain wiring 81 of the switching TFT 1301 at this point.

Note that the above switching TFT 1301 and the current control TFT 1302may be formed in accordance with a known method of manufacturing areverse stagger type TFT. Further, similar materials used incorresponding portions of the top gate type TFTs of embodiment 1 can beused for the materials of each portion (such as wirings, insulatingfilms, and active layers) formed in the above TFTs. Note that thechannel protecting films 78 a, 78 b, and 87, which are not in theconstitution of the top gate type TFT, may be formed by an insulatingfilm containing silicon. Furthermore, regarding the formation ofimpurity regions such as the source regions, the drain regions, and theLDD regions, they may be formed by using a photolithography techniqueand individually changing the impurity concentration.

When the TFTs are completed, a pixel having an EL element 1303 in whichthe first passivation film 41, the insulating film (leveling film) 44,the second passivation film 49, the pixel electrode (anode) 46, the ELlayer 47, the MgAg electrode (cathode) 45, the aluminum electrode(protecting film) 48, and the third passivation film 50 are formed inorder, is completed. Embodiment 1 may be referred to with respect tomanufacturing processes and materials for the above.

Note that it is possible to freely combine the constitution ofembodiment 5 with the constitution of any of embodiments 2 to 4.

Embodiment 6

It is effective to use a material having a high thermal radiatingeffect, similar to that of the first passivation film 41 and the secondpassivation film 49, as the base film formed between the active layerand the substrate in the structures of FIG. 5C of embodiment 1 orFIG. 1. In particular, a large amount of current flows in the currentcontrol TFT, and therefore heat is easily generated, and deteriorationdue to self generation of heat can become a problem. Thermaldeterioration of the TFT can be prevented by using the base film ofembodiment 6, which has a thermal radiating effect, for this type ofcase.

The effect of protecting from the diffusion of mobile ions from thesubstrate is also very important, of course, and therefore it ispreferable to use a lamination structure of a compound including Si, Al,N, O, and M, and an insulating film containing silicon, similar to thefirst passivation film 41.

Note that it is possible to freely combine the constitution ofembodiment 6 with the constitution of any of embodiments 1 to 5.

Embodiment 7

When the pixel structure shown in embodiment 3 is used, the lightemitted from the EL layer is radiated in the direction opposite to thesubstrate, and therefore it is not necessary to pay attention to thetransmissivity of materials, such as the insulating film, which existbetween the substrate and the pixel electrode. In other words, materialswhich have a somewhat low transmissivity can also be used.

It is therefore advantageous to use a carbon film, such as one referredto as a diamond thin film, a diamond-like carbon film, or an amorphouscarbon film, as the base film 12 or the first passivation film 41. Inother words, because it is not necessary to worry about lowering thetransmissivity, the film thickness can be set thick, to between 100 and500 nm, and it is possible to have a very high thermal radiating effect.

Regarding the use of the above carbon films in the second passivationfilm 49, note that a reduction in the transmissivity must be avoided,and therefore it is preferable to set the film thickness to between 5and 100 nm.

Note that, in embodiment 7, it is effective to laminate the carbon filmswith another insulating film when a carbon film is used in any of thebase film 12, the first passivation film 41 and the second passivationfilm 49.

In addition, embodiment 7 is effective when the pixel structure shown inembodiment 3 is used, and for other constitutions, it is possible tofreely combine the constitution of embodiment 7 with the constitution ofany of embodiments 1 to 6.

Embodiment 8

The amount of the off current value in the switching TFT in the pixel ofthe EL display device is reduced by using a multi-gate structure for theswitching TFT, and the present invention is characterized by theelimination of the need for a storage capacitor. This is a device formaking good use of the surface area, reserved for the storage capacitor,as an emitting region.

However, even if the storage capacitor is not completely eliminated, aneffect of increasing the effective emitting surface area, by the amountthat the exclusive surface area is made smaller, can be obtained. Inother words, the object of the present invention can be sufficientlyachieved by reducing the value of the off current by using a multi-gatestructure for the switching TFT, and by only shrinking the exclusivesurface area of the storage capacitor.

It is therefore possible to use a pixel structure such as that shown inFIG. 14. Note that, when necessary, the same symbols are used in FIG. 14as in FIG. 1.

The different point between FIG. 14 and FIG. 1 is the existence of astorage capacitor 1401 connected to the switching TFT. The storagecapacitor 1401 is formed by a semiconductor region (lower electrode)extended from the drain region 14 of the switching TFT 201, the gateinsulating film 18, and a capacitor electrode (upper electrode) 1403.The capacitor electrode 1403 is formed at the same time as the gateelectrodes 19 a, 19 b, and 35 of the TFT.

A top view is shown in FIG. 15A. The cross sectional diagram taken alongthe line A-A′ in the top view of FIG. 15A corresponds to FIG. 14. Asshown in FIG. 15A, the capacitor electrode 1403 is electricallyconnected to the source region 31 of the current control TFT through aconnecting wiring 1404 which is electrically connected to the capacitorelectrode 1403. Note that the connection wiring 1404 is formed at thesame time as the source wirings 21 and 36, and the drain wirings 22 and37. Furthermore, FIG. 15B shows the circuit constitution of the top viewshown in FIG. 15A.

Note that the constitution of embodiment 8 can be freely combined withthe constitution of any of embodiments 1 to 7. In other words, only thestorage capacitor is formed within the pixel, no limitations are addedwith regard to the TFT structure or the EL layer materials.

Embodiment 9

Laser crystallization is used as the means of forming the crystallinesilicon film 302 in embodiment 1, and a case of using a different meansof crystallization is explained in embodiment 9.

After forming an amorphous silicon film in embodiment 9, crystallizationis performed using the technique recorded in Japanese Patent ApplicationLaid-open No. Hei 7-130652. The technique recorded in the above patentapplication is one of obtaining a crystalline silicon film having goodcrystallinity by using an element such as nickel as a catalyst forpromoting crystallization.

Further, after the crystallization process is completed, a process ofremoving the catalyst used in the crystallization may be performed. Inthis case, the catalyst may be gettered using the technique recorded inJapanese Patent Application Laid-open No. Hei 10-270363 or JapanesePatent Application Laid-open No. Hei 8-330602.

In addition, a TFT may be formed using the technique recorded in thespecification of Japanese Patent Application Laid-open No. Hei 11-076967by the applicant of the present invention.

The processes of manufacturing shown in embodiment 1 are one embodimentof the present invention, and provided that the structure of FIG. 1 orof FIG. 5C of embodiment 1 can be realized, then other manufacturingprocess may also be used without any problems, as above.

Note that it is possible to freely combine the constitution ofembodiment 9 with the constitution of any of embodiments 1 to 8.

Embodiment 10

In driving the EL display device of the present invention, analogdriving can be performed using an analog signal as an image signal, anddigital driving can be performed using a digital signal.

When analog driving is performed, the analog signal is sent to a sourcewiring of a switching TFT, and the analog signal, which contains grayscale information, becomes the gate voltage of a current control TFT.The current flowing in an EL element is then controlled by the currentcontrol TFT, the EL element emitting intensity is controlled, and grayscale display is performed. In this case, it is preferable to operatethe current control TFT in a saturation region. In other words, it ispreferable to operate the TFT within the conditions of|V_(ds)|>|V_(gs)−V_(th)|. Note that V_(ds) is the voltage differencebetween a source region and a drain region, V_(gs) is the voltagedifference between the source region and a gate electrode, and V_(th) isthe threshold voltage of the TFT.

On the other hand, when digital driving is performed, in contrast to theanalog type gray scale display, gray scale display is performed by timedivision driving (time ratio gray scale driving) or surface area ratiogray scale driving. Namely, by regulating the length of the emissiontime or the ratio of emitting surface area, color gray scales can bemade to be seen visually as changing. In this case, it is preferable tooperate the current control TFT in the linear region. In other words, itis preferable to operate the TFT within the conditions of|V_(ds)|<|V_(gs)−V_(th)|.

The EL element has an extremely fast response speed in comparison to aliquid crystal element, and therefore it is possible to have high speeddriving. Therefore, the EL element is one which is suitable for timeratio gray scale driving, in which one frame is partitioned into aplural number of subframes and then gray scale display is performed.Furthermore, it has the advantage of the period of one frame beingshort, and therefore the amount of time for which the gate voltage ofthe current control TFT is maintained is also short, and a storagecapacitor can be made smaller or eliminated.

The present invention is a technique related to the element structureand therefore any method of driving it may thus be used.

Embodiment 11

In embodiment 11, examples of the pixel structure of the EL displaydevice of the present invention are shown in FIGS. 21A and 21B. Notethat in embodiment 11, reference numeral 4701 denotes a source wiring ofa switching TFT 4702, reference numeral 4703 denotes a gate wiring ofthe switching TFT 4702, reference numeral 4704 denotes a current controlTFT, 4705 denotes an electric current supply line, 4706 denotes a powersource control TFT, 4707 denotes a power source control gate wiring, and4708 denotes an EL element. Japanese Patent Application No. Hei11-341272 may be referred to regarding the operation of the power sourcecontrol TFT 4706.

Further, in embodiment 11 the power source control TFT 4706 is formedbetween the current control TFT 4704 and the EL element 4708, but astructure in which the current control TFT 4704 is formed between thepower source control TFT 4706 and the EL element 4708 may also be used.In addition, it is preferable for the power source control TFT 4706 tohave the same structure as the current control TFT 4704, or for both tobe formed in series by the same active layer.

FIG. 21A is an example of a case in which the electric current supplyline 4705 is common between two pixels. Namely, this is characterized inthat the two pixels are formed having linear symmetry around theelectric current supply line 4705. In this case, the number of electriccurrent supply lines can be reduced, and therefore the pixel portion canbe made even more high precision.

Furthermore, FIG. 21B is an example of a case in which an electriccurrent supply line 4710 is formed parallel to the gate wiring 4703, andin which a power source control gate wiring 4711 is formed parallel tothe source wiring 4701. Note that in FIG. 23B, the structure is formedsuch that the electric current supply line 4710 and the gate wiring 4703do not overlap, but provided that both are wirings formed on differentlayers, then they can be formed to overlap, sandwiching an insulatingfilm. In this case, the exclusive surface area of the electric currentsupply line 4710 and the gate wiring 4703 can be shared, and the pixelsection can be made even more high precision.

Embodiment 12

In embodiment 12, examples of the pixel structure of the EL displaydevice of the present invention are shown in FIGS. 22A and 22B. Notethat in embodiment 12, reference numeral 4801 denotes a source wiring ofa switching TFT 4802, reference numeral 4803 denotes a gate wiring ofthe switching TFT 4802, reference numeral 4804 denotes a current controlTFT, 4805 denotes an electric current supply line, 4806 denotes anerasure TFT, 4807 denotes an erasure gate wiring, and 4808 denotes an ELelement. Japanese Patent Application Laid-open No. Hei 11-338786 may bereferred to regarding the operation of the erasure TFT 4806.

The drain of the erasure TFT 4806 is connected to a gate of the currentcontrol TFT 4804, and it becomes possible to forcibly change the gatevoltage of the current control TFT 4804. Note that an n-channel TFT or ap-channel TFT may be used for the erasure TFT 4806, but it is preferableto make it the same structure as the switching TFT 4802 so that the offcurrent value can be made smaller.

FIG. 22A is an example of a case in which the electric current supplyline 4805 is common between two pixels. Namely, this is characterized inthat the two pixels are formed having linear symmetry around theelectric current supply line 4805. In this case, the number of electriccurrent supply lines can be reduced, and therefore the pixel section canbe made even more high precision.

In addition, FIG. 22B is an example of a case in which an electriccurrent supply line 4810 is formed parallel to the gate wiring 4803, andin which an erasure gate wiring 4811 is formed parallel to the sourcewiring 4801. Note that in FIG. 22B, the structure is formed such thatthe electric current supply line 4810 and the gate wiring 4803 do notoverlap, but provided that both are wirings formed on different layers,then they can be formed to overlap, sandwiching an insulating film. Inthis case, the exclusive surface area of the electric current supplyline 4810 and the gate wiring 4803 can be shared, and the pixel sectioncan be made even more high precision.

Embodiment 13

The EL display device of the present invention may have a structure inwhich several TFTs are formed within a pixel. In embodiments 11 and 12,examples of forming three TFTs are shown, but from 4 to 6 TFTs may alsobe formed. It is possible to implement the present invention withoutplacing any limitations on the structure of the pixels of the EL displaydevice.

Embodiment 14

An example of using a p-channel TFT as the current control TFT 202 ofFIG. 1 is explained in embodiment 14. Note that other portions are thesame as those of FIG. 1, and therefore a detailed explanation of theother portions is omitted.

A cross sectional structure of the pixel of embodiment 14 is shown inFIG. 23. Embodiment 1 may be referred to for a method of manufacturingthe p-channel TFT used in embodiment 14. An active layer of thep-channel TFT comprises a source region 2801, a drain region 2802, and achannel forming region 2803, and the source region 2801 is connected tothe source wiring 36, and the drain region 2802 is connected to thedrain wiring 37.

For cases in which the anode of an EL element is connected to thecurrent control TFT, it is preferable to use the p-channel TFT as thecurrent control TFT.

Note that it is possible to implement the constitution of embodiment 14by freely combining it with the constitution of any of embodiments 1 to13.

Embodiment 15

By using an EL material in which phosphorescence from a triplet stateexciton can be utilized in light emission in embodiment 15, the externalemission quantum efficiency can be increased by a great amount. By doingso, it becomes possible to make the EL element into a low powerconsumption, long life, and low weight EL element.

Reports of utilizing triplet state excitons and increasing the externalemission quantum efficiency is shown in the following papers.

-   Tsutsui, T., Adachi, C., and Saito, S., Photochemical Processes in    Organized Molecular Systems, Ed. Honda, K., (Elsevier Sci. Pub.,    Tokyo, 1991), p. 437.

The molecular formula of the EL material (coumarin pigment) reported inthe above paper is shown below.

-   Baldo, M. A., O'Brien. D. F., You, Y., Shoustikov, A Sibley, S.,    Thompson, M. E., and Forrest, S. R., Nature 395 (1998) p. 151.

The molecular formula of the EL material (Pt complex) reported in theabove paper is shown below.

-   -   [formula 9]

-   Baldo, M. A., Lamansky, S., Burrows, P. E., Thompson, M. E., and    Forrest, S. R., Appl. Phys. Lett., 75 (1999) p. 4.

-   Tsutui, T., Yang, M. J., Yahiro, M., Nakamura, K., Watanabe, T.,    Tsuji, T., Fukuda, Y., Wakimoto, T., Mayaguchi, S., Jpn. Appl.    Phys., 38 (12B) (1999) L1502.

The molecular formula of the EL material (Ir complex) reported in theabove paper is shown below.

Provided that the phosphorescence emission from triplet state excitonscan be utilized, then in principle it is possible to realize an externalemission quantum efficiency which is 3 to 4 times higher than that forcases of using the fluorescence emission from singlet state excitons.Note that it is possible to implement the constitution of embodiment 15by freely combining it with the constitution of any of embodiments 1 to13.

Embodiment 16

In embodiment 1 it is preferable to use an organic EL material as an ELlayer, but the present invention can also be implemented using aninorganic EL material. However, current inorganic EL materials have anextremely high driving voltage, and therefore a TFT which has voltageresistance characteristics that can withstand the driving voltage mustbe used in cases of performing analog driving.

Alternatively, if inorganic EL materials having lower driving voltagesthan conventional inorganic EL materials are developed, then it ispossible to apply them to the present invention.

Further, it is possible to freely combine the constitution of embodiment16 with the constitution of any of embodiments 1 to 14.

Embodiment 17

An active matrix type EL display device (EL module) formed byimplementing the present invention has superior visibility in brightlocations in comparison to a liquid crystal display device because it isa self-emitting type device. It therefore has a wide range of uses as adirect-view type EL display (indicating a display incorporating an ELmodule).

Note that a wide viewing angle can be given as one advantage which theEL display has over a liquid crystal display. The EL display of thepresent invention may therefore be used as a display (display monitor)having a diagonal equal to 30 inches or greater (typically equal to 40inches or greater) for appreciation of TV broadcasts by large screen.

Further, not only can it be used as an EL display (such as a personalcomputer monitor, a TV broadcast reception monitor, or an advertisementdisplay monitor), it can be used as a display for various electronicdevices.

The following can be given as examples of such electronic devices: avideo camera; a digital camera; a goggle type display (head mounteddisplay): a car navigation system; a personal computer; a portableinformation terminal (such as a mobile computer, a mobile telephone, oran electronic book); and an image playback device using a recordingmedium (specifically, a device which performs playback of a recordingmedium and is provided with a display which can display those images,such as a compact disk (CD), a laser disk (LD), or a digital video disk(DVD)). Examples of these electronic devices are shown in FIGS. 16A to16F.

FIG. 16A is a personal computer, comprising a main body 2001, a casing2002, a display portion 2003, and a keyboard 2004. The present inventioncan be used in the display portion 2003.

FIG. 16B is a video camera, comprising a main body 2101, a displayportion 2102, an audio input portion 2103, operation switches 2104, abattery 2105, and an image receiving portion 2106. The present inventioncan be used in the display portion 2102.

FIG. 16C is a goggle display, comprising a main body 2201, a displayportion 2202, and an arm portion 2203. The present invention can be usedin the display portion 2202.

FIG. 16D is a mobile computer, comprising a main body 2301, a cameraportion 2302, an image receiving portion 2303, operation switches 2304,and a display portion 2305. The present invention can be used in thedisplay portion 2305.

FIG. 16E is an image playback device (specifically, a DVD playbackdevice) provided with a recording medium, comprising a main body 2401, arecording medium (such as a CD, an LD, or a DVD) 2402, operationswitches 2403, a display portion (a) 2404, and a display portion (b)2405. The display portion (a) is mainly used for displaying imageinformation, and the image portion (b) is mainly used for displayingcharacter information, and the present invention can be used in theimage portion (a) and in the image portion (b). Note that the presentinvention can be used as an image playback device provided with arecording medium in devices such as a CD playback device and gameequipment.

FIG. 16F is an EL display, containing a casing 2501, a support stand2502, and a display portion 2503. The present invention can be used inthe display portion 2503. The EL display of the present invention isespecially advantageous for cases in which the screen is made large, andis favorable for displays having a diagonal greater than or equal to 10inches (especially one which is greater than or equal to 30 inches).

Furthermore, if the emission luminance of EL materials becomes higher infuture, then it will become possible to use the present invention in afront type or a rear type projector.

The above electronic devices are becoming more often used to displayinformation provided through an electronic transmission circuit such asthe Internet or CATV (cable television), and in particular,opportunities for displaying animation information are increasing. Theresponse speed of EL materials is extremely high, and therefore ELdisplays are suitable for performing this type of display.

The emitting portion of the EL display device consumes power, andtherefore it is preferable to display information so as to have theemitting portion become as small as possible. Therefore, when using theEL display device in a display portion which mainly displays characterinformation, such as a portable information terminal, in particular, aportable telephone of a car audio system, it is preferable to drive itby setting non-emitting portions as background and forming characterinformation in emitting portions.

FIG. 20A is a portable telephone, comprising a main body 2601, an audiooutput portion 2602, an audio input portion 2603, a display portion2604, operation switches 2605, and an antenna 2606. The EL displaydevice of the present invention can be used in the display portion 2604.Note that by displaying white characters in a black background in thedisplay portion 2604, the power consumption of the portable telephonecan be reduced.

FIG. 20B is an on-board audio system (car audio system), containing amain body 2701, a display portion 2702, and operation switches 2703 and2704. The EL display device of the present invention can be used in thedisplay portion 2702. Furthermore, an on-board audio system is shown inembodiment 17, but a desktop type audio system may also be used. Notethat by displaying white characters in a black background in the displayportion 2702, the power consumption can be reduced.

The range of applications of the present invention is thus extremelywide, and it is possible to apply the present invention to electronicdevices in all fields. Furthermore, the electronic devices of embodiment17 can be realized by using any constitution of any combination ofembodiments 1 to 16.

By using the present invention, it becomes possible to form a pixel inwhich TFTs, having optimal performance in response to the specificationsrequired by elements, are formed on the same substrate, and theoperation performance and reliability of an active matrix type ELdisplay device can be greatly increased.

Furthermore, by using this type of EL display device as a display, itbecomes possible to produce applied products (electronic equipment)having good image quality and durability (high reliability).

1. A light emitting device comprising: a substrate; a light emittingelement over the substrate, the light emitting element comprising: afirst electrode over the substrate; an electroluminescent layer over thefirst electrode, the electroluminescent layer including an organiccompound; and a second electrode over the electroluminescent layer; anda layer over the light emitting element, the layer including at leastone element selected from the group consisting of boron, carbon, andnitrogen, and at least one element selected from the group consisting ofaluminum, silicon, and phosphorous, wherein the second electrode arecapable of transmitting light emitted from the electroluminescent layer.2. The light emitting device according to claim 1, wherein the secondelectrode is an anode.
 3. The light emitting device according to claim1, wherein the layer possess a heat radiation effect.
 4. The lightemitting device according to claim 1, wherein the layer is an insulatinglayer containing silicon.
 5. The light emitting device according toclaim 1, wherein the electroluminescent layer comprises phosphorescencematerial.
 6. The light emitting device according to claim 1, furthercomprising a protecting electrode between the layer and the secondelectrode.
 7. The light emitting device according to claim 1, furthercomprising a TFT over the substrate, wherein the TFT is electricallyconnected to the light emitting element.
 8. A semiconductor devicecomprising the light emitting device of claim 1, wherein thesemiconductor device is at least one member selected from a groupconsisting of a video camera, a digital camera, a goggle type display, acar navigation system, a personal computer, and a portable informationterminal.
 9. A light emitting device comprising: a substrate; atransistor over the substrate; a light emitting element over thesubstrate, the light emitting element comprising: a first electrode overthe substrate; an electroluminescent layer over the first electrode, theelectroluminescent layer including an organic compound; and a secondelectrode over the electroluminescent layer; and a layer over the lightemitting element, the layer including at least one element selected fromthe group consisting of boron, carbon, and nitrogen, and at least oneelement selected from the group consisting of aluminum, silicon, andphosphorous, wherein light emitted from the electroluminescent layer isemitted in a direction opposite to the substrate.
 10. The light emittingdevice according to claim 9, wherein the second electrode is an anode.11. The light emitting device according to claim 9, wherein the layerpossess a heat radiation effect.
 12. The light emitting device accordingto claim 9, wherein the layer is an insulating layer containing silicon.13. The light emitting device according to claim 9, wherein theelectroluminescent layer comprises phosphorescence material.
 14. Thelight emitting device according to claim 9, further comprising aprotecting electrode between the layer and the second electrode.
 15. Thelight emitting device according to claim 9, further comprising a TFTover the substrate, wherein the TFT is electrically connected to thelight emitting element.
 16. A semiconductor device comprising the lightemitting device of claim 9, wherein the semiconductor device is at leastone member selected from a group consisting of a video camera, a digitalcamera, a goggle type display, a car navigation system, a personalcomputer, and a portable information terminal.
 17. A light emittingdevice comprising: a substrate; a light emitting element over thesubstrate, the light emitting element comprising: a first electrode overthe substrate; an electroluminescent layer over the first electrode, theelectroluminescent layer including an organic compound; and a secondelectrode over the electroluminescent layer; and a layer over the lightemitting element, the layer including at least one element selected fromthe group consisting of boron, carbon, and nitrogen, and at least oneelement selected from the group consisting of aluminum, silicon, andphosphorous, wherein the first electrode are capable of transmittinglight emitted from the electroluminescent layer.
 18. The light emittingdevice according to claim 17, wherein the second electrode is a cathode.19. The light emitting device according to claim 17, wherein the layerpossess a heat radiation effect.
 20. The light emitting device accordingto claim 17, wherein the layer is an insulating layer containingsilicon.
 21. The light emitting device according to claim 17, whereinthe electroluminescent layer comprises phosphorescence material.
 22. Thelight emitting device according to claim 17, further comprising aprotecting electrode between the layer and the second electrode.
 23. Thelight emitting device according to claim 17, further comprising a TFTover the substrate, wherein the TFT is electrically connected to thelight emitting element.
 24. A semiconductor device comprising the lightemitting device of claim 17, wherein the semiconductor device is atleast one member selected from a group consisting of a video camera, adigital camera, a goggle type display, a car navigation system, apersonal computer, and a portable information terminal.
 25. A lightemitting device comprising: a substrate; a light emitting element overthe substrate, the light emitting element comprising: a first electrodeover the substrate; an electroluminescent layer over the firstelectrode, the electroluminescent layer including an organic compound;and a second electrode over the electroluminescent layer; and a layerover the light emitting element, the layer including at least oneelement selected from the group consisting of boron, carbon, andnitrogen, and at least one element selected from the group consisting ofaluminum, silicon, and phosphorous, wherein light emitted from theelectroluminescent layer is emitted in a direction to the substrate. 26.The light emitting device according to claim 25, wherein the secondelectrode is a cathode.
 27. The light emitting device according to claim25, wherein the layer possess a heat radiation effect.
 28. The lightemitting device according to claim 25, wherein the layer is aninsulating layer containing silicon.
 29. The light emitting deviceaccording to claim 25, wherein the electroluminescent layer comprisesphosphorescence material.
 30. The light emitting device according toclaim 25, further comprising a protecting electrode between the layerand the second electrode.
 31. The light emitting device according toclaim 25, further comprising a TFT over the substrate, wherein the TFTis electrically connected to the light emitting element.
 32. Asemiconductor device comprising the light emitting device of claim 25,wherein the semiconductor device is at least one member selected from agroup consisting of a video camera, a digital camera, a goggle typedisplay, a car navigation system, a personal computer, and a portableinformation terminal.